The present invention relates to cloning vectors, cloning methods and cloning kits. More particularly, the present invention relates to a cloning vector comprising a nucleic acid sequence encoding a polypeptide which when expressed is lethal for a bacterial host cell and at least one cloning site wherein the insertion of a foreign nucleic acid insert in the cloning site causes a disruption of the expression of the lethal polypeptide.
For many molecular biology applications DNA fragments are inserted into cloning vectors, transformed into bacterial cells and plated onto selection medium for individual colony isolation. Frequently, however, problems occur because host cells containing the desired recombinant vector containing the foreign DNA are only obtained with low frequency, particularly when blunt-ended DNA fragments are inserted into the vectors.
U.S. Pat. No. 5,843,656 discloses a recombinant clone selection system for expression in a host cell comprising a repressor gene coding for a repressor, a promoter for promoting the expression of the repressor gene, a restriction endonuclease cleavage insertion site located within the repressor gene or its associated promoter such that when a foreign nucleic acid is inserted at the insertion site, expression of the repressor gene is insertionally inactivated, a surface-expressed moiety gene and an operator functionally linked to the expression of the surface-expressed moiety gene such that when the repressor is bound to the operator expression of the surface-expressed moiety gene is repressed. Transformed host cells containing the foreign nucleic acids may be isolated via the expression of the surface-expressed moiety gene. The method, however, is elaborate and requires compared to standard cloning methods a considerably larger amount of time.
U.S. Pat. No. 6,891,687 claims a vector for cloning a DNA sequence into adenylate-cyclase-positive host cells, the vector containing a selection gene comprising a modified CAP gene which codes for a modified CAP protein with reduced DNA binding specifity compared to a wild-type CAP protein, the expression of which selection gene is lethal in a portion of the host cells, wherein a restriction site for the DNA sequence is located within the selection gene and upon insertion of the DNA sequence at the restriction site, the expression of the selection gene in the host cells is prevented, wherein the modified CAP gene is modified such that the glutamic acid at position 181 of the wild-type CAP protein is substituted by glutamine. A disadvantage of this system is that it is restricted to adenylate-cyclase-positive host cells.
The problem underlying the present invention was thus to provide a new cloning system avoiding the disadvantages of the prior art. This problem is solved by providing a cloning vector comprising (a) a nucleic acid sequence encoding a polypeptide which is lethal for a bacterial host cell wherein the nucleic acid sequence is operatively linked to a regulatable expression control sequence, wherein said polypetide is selected from the Anabaena flos-aquae GvpA protein or a polypeptide which is homologous thereto, and (b) at least one cloning site wherein insertion of a foreign nucleic acid insert in the cloning site causes a disruption of the expression of the lethal polypeptide.
The cloning vector of the present invention is a tool for high-efficient cloning of DNA fragments and high-level expression of proteins in bacterial hosts, e.g. E. coli. The positive selection that makes the cloning so easy is based on the nucleic acid sequence encoding for a small protein which is located on the vector. Over expression of this polypeptide is toxic for E. coli and leads to cell death of bacteria carrying the nucleic acid sequence. When a DNA fragment is inserted into the cloning site of the vector, which may be located within the protein coding sequence or between the expression control sequence and the protein coding sequence, the expression of the lethal gene is disrupted. Thus, only transformants with insert-containing vectors will grow, whereas transformants containing the vector without insert cannot grow. The vector is particularly useful for the construction of nucleic acid, e.g. cDNA or genomic libraries, e.g. for sequencing projects and the cloning of PCR products.
Preferably, the vector of the present invention is a bacterial vector, i.e. it contains nucleic acid sequences which allow a propagation in prokaryotic host cells, particularly E. coli. Thus, a vector comprises at least one origin of replication which may be a bacterial origin of replication such as colE1 or p15A or a bacteriophage origin such as the f1 origin. In a particularly preferred embodiment, the vector comprises at least two origins of replication, wherein at least one origin is a bacterial origin and at least one orgin is a bacteriophage origin. Furthermore, it is preferred that the vector comprises a selection marker gene, which allows selection on transformants containing the vector. The selection marker gene may be an antibiotic resistance gene such as the ampicillin, kanamycin or tetracyclin resistance gene, In a particularly preferred embodiment the antibiotic resistance gene is the ampicillin resistance gene, i.e. the beta-lactamase gene.
An essential feature of the present invention is the presence of a nucleic acid sequence in the vector encoding a polypeptide which is conditionally lethal for the bacterial host cell. This nucleic acid sequence is operatively linked to a suitable expression control sequence, which is regulatable in a manner which allows propagation of the vector under conditions, wherein the expression control sequence is substantially inactive. Preferably, the expression control sequence is regulatable by a repressor. A specific example is a phage lambda promoter, e.g. the PL or PR promoter including operator sequences which allow effective binding of lambda repressor and thus substantially inactivating the expression control sequence under appropriate conditions, e.g. in a host cell, which produces the lambda repressor in sufficient amount. Furthermore, the expression control sequence may comprise a translation enhancer, e.g. the T7 translation enhancer to allow effective translation of any heterologous nucleic acid sequence which is inserted into the cloning site.
The vector may also contain at least one nucleotide sequence which is complementary to a sequencing and/or amplification primer. These complementary nucleotide sequences are preferably located adjacent to the cloning site and thus allow efficient sequencing and/or amplification of any foreign nucleic acid fragment which is inserted in the cloning site. Moreover, the vector may be a shuttle vector, which allows propagation in different host cells, e.g. it may additionally comprise nucleic acid sequences which allow a propagation in eukaryotic host cells.
The lethal polypeptide is selected from the Anabaena flos-aquae GvpA protein (Genbank accession No. M32060) or a polypeptide homologous thereto, which may be a protein from Fremyella diplosiphon (Genbank accession No. P07060), a protein from Calothrix (Genbank accession No. AAB23332), a protein from Pseudoanabaena spec. (Genbank accesssion No. P22453), a protein from Planktothrix rubescens (Genbank accession No. CAB59543 and CAB59546), a protein from Aphanizomenon flos-aquae (Genbank accession No. SVFZ), a protein from Microcystis spec. (Genbank accession No. P08412), a protein from Oscillatoria agardhii (Genbank accession No. P80996), a protein from Thiocapsa pendens (Genbank accession No. P80998), a protein from Amoebobacter pendens (Genbank accession No. AAB23337), a protein from Bacillus megaterium (Genbank accession No. AAC38419, AAC38416 and AF053765), a protein from Spirulina spec. (Genbank accession No. P80997), a protein from Haloferax mediterranei (Genbank accession No. P23761 and Q02235), a protein from Halobacterium halobium (Genbank accession No. P08959, S07323, P08959 and P24374), a protein from Dactylococcopsis salina (Genbank accession No. AAB23336), a protein from Halorubrum vacuolatum (Genbank accession No. CAA69881), a protein from Streptomyces coelicolor (Genbank accession No. CAB61167, CAB61172 and CAA22042) and a protein from Halobacterium salinarium (Genbank accession No. P33956).
The present invention encompasses a nucleic acid sequence encoding a conditionally lethal polypeptide which is homologous to the GvpA protein from Anabaena flos-aquae. The homologous polypeptide preferably has an identity of more than 60%, preferably more than 75% and particulary preferred more than 90% to the GvpA protein and is, when expressed under non-repressed conditions, lethal for a bacterial host.
The identity is determined on protein level as follows:       I    =          n      L        ,
wherein
I represents the identity in percent
n represents the number of different amino acids between a test sequence and the GvpA sequence and
L is the length of the GvpA sequence to be compared with a test sequence.
The nucleic acid sequence encoding the Anabaena flos-aquae GvpA protein and the corresponding polypeptide sequence are described in Genbank accession No. M322060. In a particularly preferred embodiment of the present invention the lethal polypeptide is encoded by (a) the nucleotide sequence encoding the Anabaena flos-aquae GvpA protein, (b) a nucleotide sequence corresponding to the sequence of (a) within the scope of the degeneracy of the genetic code or (c) a nucleotide sequence hybridizing under stringent conditions with the sequence of (a) and/or (b).
The term xe2x80x9chybridization under stringent conditionsxe2x80x9d is defined according to Sambrook et al., Molecular Cloning, A Laboratory Manual, Cold Spring Harbor, Laboratory Press (1989), 1.101-1.104. Preferably, hybridization under stringent conditions means that after washing for 1 h with 1xc3x97SSC and 0.1% SDS at 50xc2x0 C., preferably at 55xc2x0 C., more preferably at 62xc2x0 C. and most preferably at 68xc2x0 C., particularly for 1 h in 0.2xc3x97SSC and 0.1% SDS at 50xc2x0 C., preferably at 55xc2x0 C., more preferably at 62xc2x0 C. and most preferably at 68xc2x0 C., a positive hybridization signal is observed. A nucleotide sequence which hybridizes under the above washing conditions with the nucleotide sequence coding for the gvpA gene of Anabaena flos-aquae (Genbank accession No. M32060) or a nucleotide sequence corresponding thereto in the scope of the degeneracy of the genetic code is encompassed by the present invention.
The vector also comprises a cloning site which allows the insertion of foreign nucleic acid fragments. The position of the cloning site is such that insertion of foreign nucleic acid fragments causes a disruption, particularly a substantially complete inactivation of the expression of the lethal polypeptide. Thus, the cloning site may be located within the nucleic acid sequence encoding the lethal polypeptide or between the expression control sequence and the start of the polypeptide sequence. More preferably, the cloning site is located adjacent to the 5xe2x80x2-region of the coding sequence. Further, it is preferred that the cloning site is a multiple cloning site comprising recognition sites for a plurality of restriction enzymes, which e.g. may be selected from the group consisting of Ncol, Kpnl, Acc65l, Xmal, Smal, Smal, EcoRI, Xhol, NotI, EcoRV and SaII.
A further subject matter of the present invention is a cloning method comprising the steps:
(a) providing a cloning vector as described above,
(b) inserting a foreign nucleic acid molecule, which may be a blunt-ended DNA fragment, e.g. a PCR product, into the cloning site, wherein a vector containing the foreign nucleic acid molecule is obtained,
(c) transforming competent bacterial host cells, e.g. E. coli cells, with the vector obtained in step (b) and
(d) recovering transformed bacterial host cells which harbor the vector containing the foreign nucleic acid molecule.
Step (d) may comprise the culturing of transformed host cells under appropriate selection conditions, e.g. in an antibiotic containing medium, preferably a solid medium, and characterizing positive clones. When carrying out this method it was found that at least 90% of recovered transformed host cells harbor the vector containing the foreign nucleic acid molecule.
Moreover, the present invention relates to a reagent kit for cloning and optionally expressing foreign nucleic acid molecules comprising the vector as described above and further reagents. These further reagents may be selected from enzymes such as DNA ligase and/or restriciton enzymes, e.g. T4 DNA ligase and restriction endonuclease Smal, buffers, e.g. enzyme buffers and sequencing or amplification primers. Furthermore, the kit may comprise competent bacterial host cells, e.g. E. coli cells.
Further, the invention shall be explained by the following figures and examples:
FIG. 1 shows a schematic image of the vector pCorrectClone according to the Invention.
SEQ ID NO 1 shows the nucleotide sequence of the vector pCorrectClone including the gvpA sequence from Anabaena flos-aquae, which has been modified to include a multiple cloning site. The modified gvpA gene begins at position 251 (ATG) and ends at position 505 (GCT).